1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly, it relates to an acceleration sensor which is formed by a bimorph piezoelectric element.
2. Description of the Background Art
In general, an acceleration sensor employing a bimorph piezoelectric element is known in the art. An example of such an acceleration sensor is now described with reference to FIG. 1. This acceleration sensor 1 is provided with a sensor body 2, which is formed by a bimorph piezoelectric element.
The sensor body 2 comprises strip-shaped first and second piezoelectric ceramic plates 3 and 4, which are laminated to each other.
First and second signal electrodes 5 and 6 are formed on first major surfaces of the first and second piezoelectric ceramic plates 3 and 4, respectively. The first and second signal electrodes 5 and 6 are formed on the first major surfaces of the first and second piezoelectric ceramic plates to extend from a first edge toward a second edge, respectively, but not to reach the second edges. The first and second signal electrodes 5 and 6 are disposed opposite to each other in central regions through the first and second piezoelectric ceramic plates 3 and 4.
An intermediate electrode 7 is arranged between the first and second piezoelectric ceramic plates 3 and 4. The intermediate electrode 7 is formed to oppose the first and second signal electrodes 5 and 6, in portions where the first and second signal electrodes are opposed to each other.
The first and second piezoelectric ceramic plates 3 and 4 are uniformly polarized along arrows X and Y in FIG. 1, respectively. In other words, the first and second piezoelectric ceramic plates 4 and 5 are polarized in directions X and Y which are opposite to each other.
The sensor body 2 is held by first and second holding members 8 and 9 in portions which are close to both ends thereof. While FIG. 1 shows only the outline of the first holding member 8 in phantom lines, the first and second holding members 8 and 9 are fixed to the sensor body 2 on both ends thereof. The first and second holding members 8 and 9 are provided with cavities 8a and 9a between the portions which are fixed to the sensor body 2, respectively, in order to define spaces for allowing displacement of the sensor body 2.
The first and second holding members 8 and 9 form parts of a case holding the sensor body 2. In the structure shown in FIG. 1, a first terminal electrode 10 is formed on an outer surface of this case, i.e., on end surfaces of the sensor body 2 and the holding member 9, for example, and the first signal electrode 5 is electrically connected to the first terminal electrode 10.
Similarly, a second signal electrode 6 is electrically connected to a second terminal electrode 11 which is formed on other end surfaces of the sensor body 2 and the holding member 9.
When acceleration acts on the acceleration sensor 1 shown in FIG. 1 along arrow G, the sensor body 2 which is supported by the holding members 8 and 9 in the form of a center beam is deflected so that the first and second signal electrodes 5 and 6 detect a potential which is responsive to the acceleration on the basis of charges generated in response to the amount of deflection.
In recent years, further miniaturization is required for such an acceleration sensor. Thus, a sensor body which is incorporated in the acceleration sensor itself must also be miniaturized. If the aforementioned sensor body 2 is merely miniaturized, however, detection sensitivity is remarkably reduced due to reduction in amount of charges which are generated upon action of the acceleration G.
To this end, there has been proposed a structure of a sensor body which can be further deformed upon action of the same level of acceleration G thereby improving detection sensitivity. In an acceleration sensor 21 shown in FIG. 2, a sensor body 22 is supported in a cantilever manner between first and second holding members 18 and 19, which are similar in structure to the first and second holding members 8 and 9. The sensor body 22 is provided with first and second piezoelectric ceramic plates 23 and 24 which are pasted to each other, while first and second signal electrodes 25 and 26 are formed on first major surfaces of the piezoelectric ceramic plates 23 and 24, respectively. Further, an intermediate electrode 27 is formed between the piezoelectric ceramic plates 25 and 24.
In the acceleration sensor 21, the sensor body 22 is supported by the holding members 18 and 19 in a cantilever manner, whereby the first and second signal electrodes 25 and 26 are electrically connected to terminal electrodes 28 and 29 which are formed on first end surfaces of the holding members 18 and 19, respectively.
When acceleration acts on the acceleration sensor 21 along arrow G, the sensor body 22 which is supported in a cantilever manner can be further deformed as compared with that of the acceleration sensor 1 shown in FIG. 1. Thus, it is possible to improve detection sensitivity.
In order to manufacture the acceleration sensor 1 shown in FIG. 1, a pair of ceramic mother substrates 31 and another pair of ceramic mother substrates 32 for forming holding members are prepared as shown in FIG. 3. The ceramic mother substrates 31 are finally cut to form the first and second piezoelectric ceramic plates 3 and 4. The other pair of ceramic mother substrates 32 are adapted to form the holding members 8 and 9. Electrodes for forming signal electrodes and intermediate electrodes are provided on both major surfaces of the pair of ceramic mother substrates 51, which in turn are stacked with each other so that the other pair of ceramic mother substrates .352 are bonded to upper and lower portions thereof. An integrated structure obtained by such bonding is cut along cutting lines S shown in FIG. 3, to obtain the acceleration sensor 1 shown in FIG. 1.
On the other hand, the acceleration sensor 21 shown in FIG. 2 is manufactured by a method which is similar to the above. In this case, grooves 33 are formed in the ceramic mother substrates 31, as shown in FIG. 3. Each of these grooves 33 is adapted to release an end of the sensor body 22 in the acceleration sensor 21 shown in FIG. 2. In other words, each groove 33 is adapted to define a space for supporting the sensor body 22 in a cantilever manner. Thus, it takes much time to manufacture the acceleration sensor 21, since it is necessary to form the grooves 33 in the ceramic mother substrates 31. Further, the ceramic mother substrates 31 which are provided with the grooves 33 must be correctly aligned with each other, as well as, with the other pair of ceramic mother substrates 32 for forming holding members. Thus, it requires a long period of time to manufacture the acceleration sensor 21.
Because of the sensor body 22 being supported in a cantilever manner, impact resistance is reduced as compared with the center beam structure which results in a reduction in mechanical strength of the acceleration sensor 21. In addition, the terminal electrodes 28 and 29 for leading out the signal electrodes 25 and 26 to the exterior are exposed on the same end surface of the case in the acceleration sensor 21. In an ordinary electronic part, however, a pair of such terminal electrodes are generally exposed on different end surfaces which are opposed to each other. When a wiring board for mounting an ordinary electronic part is employed, therefore, a wiring pattern provided on the wiring board must be changed to accommodating the arrangement of the acceleration sensor 21.